re: Do i need to give my dog heartworm prevention medication

A preventative, particularly in the south, is a very good idea. However, if you don't trust your vet's word I'd go to another. FWIW, there is now a shot that can be given as opposed to the pill. At least heartworms are treatable now as opposed to years past.

Edit: Forgot to add, we switched from heartguard to the shot last year and our female turned up positive for heartworms last month when she was due for next shot. Turns out that there is a window equal to the hearworm gestation period (6 months) that can leave them open to getting them after the first shot. Male tested fine, young female will be switching shortly.